As having a high melting point and moldable in melt, polylactic acid is expected to be a practicable good biodegradable polymer. However, since its crystallization rate is low, using polylactic acid for crystallized moldings is limited. For example, when injection-molded, polylactic acid requires a long molding cycle time or heat treatment after molding, and, in addition, its deformation in molding and heat treatment is great. These are serious problems in practical use of polylactic acid.
Polyacetal resin is good as its mechanical properties and moldability are well balanced, and it is widely used for injection moldings. However, since its crystallization rate is high, the resin could not be well formed into films and fibers and its use is also limited.
Mixing two or more different polymers is widely known for polymer blends or polymer alloys, and is widely utilized for overcoming the drawbacks of the individual polymers. However, when two polymers are mixed, they form separate phases in most cases, and therefore the properties of the polymer mixtures are not good.
Though rarely, on the other hand, two polymers may form a uniform amorphous phase. Those of the type are generally said miscible or compatible with each other, and their mixtures are expected to have good properties. However, there are only a few examples of such polymers.
Regarding polymer that is miscible with (mutually make miscible with) polylactic acid, there are known only a few examples such as polyethylene glycol (e.g., Polymer 37 (26), 5849-5857 (1996)) and polymethyl methacrylate (e.g., Polymer 39(26), 6891-6897 (1998)). Regarding polymer that is miscible with polyacetal, there are known only a few examples such as polyvinylphenol (e.g., Polymer 33 (4), 760-766 (1992)).
JP-A 5-43772 discloses a resin composition prepared by adding an aliphatic polyester and a minute amount of formaldehyde to polyacetal for the purpose of making polyacetal biodegradable, concretely disclosing use of polylactic acid as one example of the aliphatic polyester, but does not disclose the miscibility of polylactic acid and polyacetal. The principal object of the invention described in the patent publication is especially to make polyacetal resin biodegradable in a composition that contains a relatively large quantity of polyacetal resin. Therefore, when it is going to obtain the composition that employ the characteristic of a polyacetal resin for ordinary use, its durability is not good and it is not practicable.
An object of the present invention is to provide a resin composition having good moldability, workability, mechanical properties, heat resistance and transparency.